Motorized screen devices are known making it possible to vary the quantity of light and energy entering through an opening. These are essentially Venetian screens, the orientation of the slats of which varies. It is, however, necessary to note that the incoming quantity follows laws that it is very difficult, if not impossible, to model correctly. It is then necessary to use a light and/or temperature sensor for each indoor space in question and to couple said sensor(s) with the automaton that will orient the slats until obtaining the required quantity of natural light. The other, more rudimentary solution consists of orienting the slats over several predefined positions and coupling that orientation with the azimuth of the sun. One thus obtains good protection in direct radiation, but a poor quality of ambient light and thermal comfort.
Also known are extensible elastic fabrics that allow more or less filtering of the light based on a deformation imposed on them. In fact, the fabrics are made by crossing weft and warp yarns. More or less significant interstices exist between each of the yarns of a same type and also between yarns of different types. These interstices define a porosity that allows light to pass in proportion to the surface area of the interstices. By using weft yarns and/or warp yarns having interlacing and a determined elasticity, it is possible to vary those interstices. The ratio of the surface area occupied by the interstices to the total surface area of the screen is referred to as the aperture ratio of the screen. In the case of an extensible elastic fabric, that aperture ratio varies with the elastic deformation of the screen.
What has been discussed here regarding fabrics can be transposed to other nonwoven screens whereof the structure has interstices or pores. In that case as well, it is possible to define an aperture ratio. When the screen is elastically extensible, the aperture ratio may vary from a minimum value, obtained with no traction force, to a maximum value, beyond which irreversible deformations may occur.
As indicated, the aperture ratio is generally used to characterize the higher or lower opacity of a woven or nonwoven screen. The aperture ratio is expressed in percentage. It is possible to obtain very low aperture ratios, i.e., close to 0% and in any case below 5%, for opaque screens, and aperture ratios of approximately 15% to 20% for traditional screens that have a sun protection effect. Past that, no further sun protection is obtained, and reference is instead made to decorative screens. For example, an aperture ratio of approximately 40% corresponds to a casement curtain. The aperture ratio may, however, also be used to characterize the capacity of the screen to allow air to pass.
It has been proposed in document EP 0,795,674 A2 to use a woven or nonwoven screen with a variable aperture ratio to conceal an opening of a building. The screen is fastened by one end to a winding tube and by the opposite end to a load bar. A mechanism makes it possible to unwind or wind the screen around the tube to close or free the opening. This mechanism further makes it possible to stretch the extensible screen, which results in increasing the translucency thereof, or to relax it, which has the opposite effect of increasing its opacity. To that end, the mechanism makes it possible to lock the position of the load bar, before acting on the roller to stretch the screen. The screen is preferably extensible in only one direction. It is preferably woven. However, the proposed mechanism is not motorized, which means that the user must measure the traction to be exerted to obtain the desired translucency. Another screen of the same type is described in document U.S. Pat. No. 3,065,785, with the same limitations.